We propose to characterize extensively the enzymes of the folic acid pathways in cultured human skin fibroblasts, peripheral blood lymphoblasts, and amniotic fluid cells. These studies aim to provide a convenient system for the detailed evaluation of the five putative inborn errors of folate metabolism with their associated mental retardation syndromes and for the exploration of the regulation of enzyme levels in human cells. We are attempting to identify humans with mutations in folate enzymes occurring naturally or produced in cell culture by chemical mutagenesis. The effects of these mutations as well as of metabolically altered growth conditions will be evaluated using both individual folate enzymes and the simultaneous levels of enzymes in the pathway as a whole.